1. Field of the Invention
This invention concerns an improved drying and curing oven for photosensitive materials applied as a coating on a substrate.
2. Description of Related Art
A typical example of a product where the oven according to this invention is used is a printed circuit board. In this case the oven is used to process fiberglass boards, having applied as a coating thereon, through silk screening, spraying, glazing or some other process, the material which will actually comprise the electric circuits, after the substrate has been unloaded off the oven and exposed to light.
Ovens of the subject type are already known, and they may be grouped in two broad families wherein the first comprises stationary ovens, while the second comprises dynamic or semi-stationary ovens.
The ovens of both families mentioned above are suitable for treating rigid or semi-rigid substrates or boards made of fiberglass, bakelite or similar materials, having applied as a coating thereon a photosensitive material, under the form of a liquid film. The applied material is made to adhere to the substrate or board, and made to dry up thereon through a suitable thermal cycle whereby it acquires all the properties required for performing the following photo-exposure process.
In the ovens of the first family, the substrates or boards are mounted on suitable carriages or racks, and they are heated by means of a flow of hot air, or through other heat sources, for a predetermined period of time. A first drawback of these ovens derives from the fact that the boards or racks must be subjected to handling, which results in a particular disadvantage at the beginning of the processing, when the material applied by coating to the board is still in a liquid phase whereby, if the board gets even slightly tilted, the material may trickle down, or cause thickness differences, or it may in any case modify the conditions under which the material was applied by coating onto the board. Therefore, the final product will be unfit for usage.
Another drawback of this type of ovens derives from the fact that the internal thermal balance of the oven is upset any time the feeding doors thereof are opened in order to load substrates or boards, and a large mass of cold air is allowed to enter into the oven. Furthermore the opening operation causes a heat loss to the outside and the danger that harmful vapours will propagate through the enclosed areas where the oven is operating, jeopardizing the operator's safety.
Some of the drawbacks mentioned above were partially done away with by providing the oven with exhausters, fans or other additional devices which, besides having proved not completely effective, were particularly expensive, resulting in non-competitive oven operations. Since the ovens of the second family are provided with means for substrate or board handling, they necessarily require the means for handling to be adjusted and set-up when the shape of the substrates or boards to be processed changes. The above causes idle times and a loss of flexibility and of dynamic response of the manufacturing lines where the oven is installed.
It should further be noted that all the handling means control devices make the oven more prone to frequent operations of maintenance, cheking and part replacement. There has now been contrived, and it is the subject of this invention, a substrate or board processing oven, as defined herein above, which does away with all the drawbacks of the ovens already known in the art.